In dimensioning of switching power supplies, especially those for industrial controls, calculation of the power consumption is a substantial expense if it is possible at all. Usually, only an estimate is possible, because the peak power consumption depends primarily on the control program that is running. Because of that, power supplies are often massively over-dimensioned, resulting, certainly, in higher costs and greater requirement for space.
Switching power supplies generally have a maximum output power which is, for instance, set at a fixed level by a current limiter, whereby the maximum output power is available as a continuous rating. In so doing, one assumes a maximum ambient temperature. At the state of the art, power supplies are known which can produce output currents or powers greater than the nominal value for short times. For instance, they can produce 3 times the nominal current for 25 ms. It has also become known that power supplies can be built so that they can provide a higher output power, e.g., 10 to 30% higher, at low ambient temperature (“derating”).
Transient overloads can also be supplied by batteries connected in parallel with the output; but that solution is quite disadvantageous because the lifetime and cost of the batteries must be considered, and a separate cutoff device to protect against deep discharge is also necessary, such as a special load circuit; and they have substantial volume and weight, aside from the cost.
The fundamental consideration of thermal capacities for fused supply of electrical loads is often recognized and is already accomplished in principle by fuses with slow response adapted to the load. For example, U.S. Pat. No. 5,283,708 A shows an electronic protection for an electrical motor connected to a three-phase line, which is to be protected against long-term load above the nominal load. In this system the current temperature of the motor can be calculated and applied for switching the motor off.
However, the problem on which the invention is based is not protection of a load and consideration of its characteristics, but provision of a switching power supply at a favorable price.
Then one objective of the invention is provision of a switching power supply with which unnecessarily high costs due to overdimensioning can be avoided and, in particular, intelligent adaptation to the existing situation, especially the load and temperature situation, is possible.
This problem is solved with a switching power supply of the type stated initially, in which, according to the invention, there is a control in which a thermal model is implemented, by means of which the temperature of at least one part can be calculated or estimated, whereby a load-dependent current value is made available to the thermal model as an electrical quantity, and the control is directed to producing at least one limiting signal when a limiting value that can be preset is reached, or a function of multiple limits, which limiting signal can be utilized in the sense of an interruption for temperature reduction, and at least one limiting signal can be sent to the control circuit, which acts on the control circuit in the sense of temperature reduction and thus power reduction.
Due to the invention, it is possible to provide a switching power supply which has only a relatively small structural size and lower costs than the usual power supplies, because it is dimensioned only for the average power consumption. In practice, one can actually reduce the dimensioning to about half the nominal power. The only added costs are those concerning the temperature monitoring.
In one practical variant it is provided that a limiting signal is used to cut off the power supply on the primary and secondary sides.
It is particularly convenient for a limiting signal to be used to control a cooling/ventilating device. In that way it is possible to prevent, or at least to delay, cutting off the power supply in the case of a transient overload.
In many cases it is desirable to use a limiting signal as an alarm signal because the user can take appropriate steps to avoid or reduce damage.
In preferred variants, it is provided that there is at least one temperature sensor to determine temperatures relevant to/for the power supply, whereby the signal from at least one temperature sensor can be used in the thermal model. In contrast to other operating parameter values, the temperature values are of direct and clear importance for an overload state of the power supply. It can be appropriate, if a temperature sensor is provided for the ambient temperature for the power supply or if a temperature sensor is provided to determine the temperature of a semiconductor component and/or its thermally relevant environment, or if a temperature sensor is provided to determine the temperature of a transformer and/or its thermally relevant environment.
On the other hand, it is appropriate in many cases if at least the output current from the power supply is provided to the thermal model as an electrical quantity, or if the primary current is provided to the thermal model as an electrical quantity. The thermal model can, from such current values, make an estimate of, for example, the chip temperature of power semiconductors.
In another variant, it can be provided for the thermal model to contain stored thermal time constants of individual parts which are taken into consideration in the calculation/estimation of component temperatures. In this way, the delay between an actual semiconductor (chip) temperature and, for instance, the housing or heat sink temperature, which is often substantial, can be taken into consideration. Here it is particularly effective if the thermal model is directed at continuously calculating the temperatures of components, considering the stored time constants.
Another variant, which can be accomplished simply, is characterized in that the thermal model contains a list of possible combinations of assignments of operating parameter values and limiting signals and the control circuit is aimed at selecting and outputting at least one limiting signal corresponding to measurements from this list.
Designs in which the control system contains at least one digital processor are particularly capable.
On the other hand, it is possible for the control system to be designed at least in part as an analog system. One can construct a thermal model, especially a simplified one, from operational amplifiers, resistances and capacitors.